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Python File Input and Output: Basics and Advanced Techniques

Learn how to open, read, write, and append files in Python. Understand different methods of reading file content, writing data to files, and handling file objects efficiently. Explore advanced file input and output techniques in Python programming.

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Python File Input and Output: Basics and Advanced Techniques

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  1. Lecture 4B Thursday, January 17, 2008 File input and outputif-then-else Genome 559: Introduction to Statistical and Computational Genomics Prof. William Stafford Noble

  2. File input and output

  3. Opening files • The open() command returns a file object. <filehandle> = open(<filename>, <access type>) • Python can read, write or append to a file: • 'r' = read • 'w' = write • 'a' = append • Create a file called “hello.txt” containing one line: “Hello, world!” >>> myFile = open("hello.txt", "r")

  4. Reading the whole file • You can read the contents of the file into a single string. >>> myString = myFile.read() >>> print myString Hello, world! >>> Why is there a blank line here?

  5. Reading the whole file • Now add a second line to your file (“How ya doin’?”) and try again. >>> myFile = open("hello.txt", "r") >>> myString = myFile.read() >>> print myString Hello, world! How ya doin'? >>>

  6. Reading the whole file • Alternatively, you can read the file into a list of strings. >>> myFile = open("hello.txt", "r") >>> myStringList = myFile.readlines() >>> print myStringList ['Hello, world!\n', "How ya doin'?\n"] >>> print myStringList[1] How ya doin'?

  7. Reading one line at a time • The readlines() command puts all the lines into a list of strings. • The readline() command returns the next line. >>> myFile = open("hello.txt", "r") >>> myString = myFile.readline() >>> print myString Hello, world! >>> myString = myFile.readline() >>> print myString How ya doin'? >>>

  8. Writing to a file • Open the file for writing or appending. >>> myFile = open("new.txt", "w") • Use the <file>.write() method. >>> myFile.write("This is a new file\n") >>> myFile.close() >>> ^D > cat new.txt This is a new file Always close a file after you are finished reading from or writing to it.

  9. Print vs write • <file>.write() does not automatically append an end-of-line character. • <file>.write() requires a string as input >>> newFile.write("foo") >>> newFile.write(1) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: argument 1 must be string or read-only character buffer, not int

  10. if-then-else

  11. The if statement >>> if (seq.startswith("C")): ... print "Starts with C" ... Starts with C >>> • A block is a group of lines of code that belong together. if (<test evaluates to true>): <execute this block of code> • In the Python interpreter, the ellipse indicates that you are inside a block. • Python uses indentation to keep track of blocks. • You can use any number of spaces to indicate blocks, but you must be consistent. • An unindented or blank line indicates the end of a block.

  12. The if statement • Try doing an if statement without indentation. >>> if (seq.startswith("C")): ... print "Starts with C" File "<stdin>", line 2 print "Starts with C" ^ IndentationError: expected an indented block

  13. Multiline blocks • Try doing an if statement with multiple lines in the block. >>> if (seq.startswith("C")): ... print "Starts with C" ... print "All right by me!" ... Starts with C All right by me!

  14. Multiline blocks • What happens if you don’t use the same number of spaces to indent the block? >>> if (seq.startswith("C")): ... print "Starts with C" ... print "All right by me!" File "<stdin>", line 4 print "All right by me!" ^ SyntaxError: invalid syntax

  15. Comparison operators • Boolean: and, or, not • Numeric: < , > , ==, !=, <>, >=, <= • String: in

  16. Examples seq = 'CAGGT' >>> if ('C' == seq[0]): ... print 'C is first' ... C is first >>> if ('CA' in seq): ... print 'CA in', seq ... CA in CAGGT >>> if (('CA' in seq) and ('CG' in seq)): ... print "Both there!" ... >>>

  17. Single equal assigns a variable name. >>> myString == "foo" Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'myString' is not defined >>> myString = "foo" >>> myString == "foo" True Double equal tests for equality. >>> if (myString = "foo"): File "<stdin>", line 1 if (myString = "foo"): ^ SyntaxError: invalid syntax >>> if (myString == "foo"): ... print "Yes!" ... Yes! Beware!= versus ==

  18. if-else statements if <test1>: <statement> else: <statement> • The else block executes only if <test1> is false. >>> if (seq.startswith('T')): ... print 'T start' ... else: ... print 'starts with', seq[0] ... starts with C >>> Evaluates to FALSE: no print.

  19. If-elif-else if <test1>: <statement> elif <test2>: <statement> else: <statement> • elif block executes if <test1> is false and then performs a second <test2>

  20. Example >>> base = 'C' >>> if (base == 'A'): ... print "adenine" ... elif (base == 'C'): ... print "cytosine" ... elif (base == 'G'): ... print "guanine" ... elif (base == 'T'): ... print "thymine" ... else: ... print "Invalid base!“ ... cytosine

  21. <file> = open(<filename>, r|w|a> <string> = <file>.read() <string> = <file>.readline() <string list> = <file>.readlines() <file>.write(<string>) <file>.close() if <test1>: <statement> elif <test2>: <statement> else: <statement> • Boolean: and, or, not • Numeric: < , > , ==, !=, <>, >=, <= • String: in, not in

  22. Sample problem #1 • Write a program read-first-line.py that takes a file name from the command line, opens the file, reads the first line, and prints the result to the screen. > python read-first-line.py hello.txt Hello, world! >

  23. Solution #1 import sys filename = sys.argv[1] myFile = open(filename, "r") firstLine = myFile.readline() myFile.close() print firstLine

  24. Sample problem #2 • Modify your program to print the first line without an extra carriage return. > python read-first-line.py hello.txt Hello, world! >

  25. Solution #2 import sys filename = sys.argv[1] myFile = open(filename, "r") firstLine = myFile.readline() firstLine = firstLine[:-1] myFile.close() print firstLine

  26. Sample problem #3 • Write a program add-two-numbers.py that reads one integer from the first line of one file and a second integer from the first line of a second file and then prints their sum. > add-two-numbers.py nine.txt four.txt 9 + 4 = 13 >

  27. Solution #3 import sys fileOne = open(sys.argv[1], "r") valueOne = int(fileOne.readline()) fileTwo = open(sys.argv[2], "r") valueTwo = int(fileTwo.readline()) print valueOne, "+", valueTwo, "=", valueOne + valueTwo

  28. Sample problem #4 • Write a program find-base.py that takes as input a DNA sequence and a nucleotide. The program should print the number of times the nucleotide occurs in the sequence, or a message saying it’s not there. > python find-base.py A GTAGCTA A occurs at position 3 > python find-base.py A GTGCT A does not occur at all Hint:S.find('G') returns -1 if it can't find the requested sequence.

  29. Solution #4 import sys base = sys.argv[1] sequence = sys.argv[2] position = sequence.find(base) if (position == -1): print base, "does not occur at all" else: print base, "occurs at position", position

  30. Reading • Chapter 13 of Learning Python (3rd edition) by Lutz.

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